Question

For instance:

static int __init hugetlb_init(){
    ......
}
module_init(hugetlb_init);

And there is no #ifdef CONFIG_xxx before it.
Does it mean that it is a manual insert module which won’t be inserted during boot?

Answer 1

But there is a CONFIG_HUGETLBFS in mm/Makefile

obj-$(CONFIG_HUGETLBFS) += hugetlb.o

given the fact that CONFIG_HUGETLBFS is boolean,

config HUGETLBFS
        bool "HugeTLB file system support"

you can either make it built-in (=y) or kick it out of kernel (=n). So, hugetlbfs can’t be a module that you can manually insert into kernel.